Conventionally, various methods have been proposed for detecting knocking (knock) that occurs in an internal combustion engine. By way of example, there is a technique of determining generation of knock when vibration intensity in the internal combustion engine is larger than a threshold value. Even when the engine is not knocking, however, intensity of noise such as vibration experienced when an intake valve or an exhaust valve closes, may be higher than the the threshold value. In such a case, false determination that knock has occurred may be made, though actually knock has not occurred. Therefore, a technique has been proposed in which whether engine knocks or not is determined based on a result of comparison between a predetermined knock waveform model and a detected vibration waveform, so as to take into consideration characteristics other than intensity, such as attenuation ratio and crank angle at which vibration occurs.
Japanese Patent Laying-Open No. 2006-226967 discloses a knock determination device for an internal combustion engine, determining whether knock has occurred or not with high accuracy. The knock determination device for an internal combustion engine includes means for detecting vibration of the internal combustion engine, extracting means for extracting vibration in a frequency band of at least one of third and fourth tangential resonance modes in the cylinder of internal combustion engine from the detected vibration, and determination means for determining whether knock has occurred in the internal combustion engine, based on the extracted vibration.
In the knock determination device disclosed in the laid-open application mentioned above, vibration in the frequency band of at least one of third and fourth tangential resonance modes, which are typical resonance modes detected particularly at the time of knocking, is extracted from the vibration of internal combustion engine and, therefore, vibration involving less noise other than knocking can be extracted. Namely, vibration characteristic of knock generation can be extracted with high accuracy. Whether knock has occurred or not is determined based on the vibration. As a result, a knock determination device for an internal combustion engine that can determine whether knock has occurred or not with high accuracy can be provided.
Representative frequency band that involves vibration detected particularly at the time of knocking includes frequency bands of first, second, third and fourth tangential resonance modes. Among these frequency bands, some frequency bands are much susceptible to superposition of vibration particular to knocking, while other frequency bands are less susceptible. By way of example, vibration in the frequency band of first tangential mode is relatively susceptible to superposition of vibration particular to knocking. It is noted, however, that the frequency band of first tangential mode also tends to be much influenced by noise other than knocking.
When the vibration in frequency band of first tangential mode is removed as in the knock determination device disclosed in the laid-open application mentioned above, it follows that the frequency band much susceptible to superposition of vibration particular to knocking is removed and, therefore, false determination of knocking possibly occurs. Further, false determination of knocking may also occur if the influence of frequency band of first tangential mode is significant in determining whether knock has occurred or not.